Trowbridge and Penrose — The Thomson Effect. 385 



From this table it appears that the Thomson Effect in ordi- 

 nary graphite is negative: that is, heat is apparently evolved 

 when the current passes from cold to hot, or the negative cur- 

 rent carries heat with it. The differences in the last columns 

 are obviously proportional to four times the Thomson Effect — 

 assuming that the effect is reversible. It also appears from the 

 tat>le thitt the effect increases as the temperature increases, 

 which is in accordance with Tail's assumption. 



These experiments were repeated with the graphite from 



'-■tlier kinds of pencils, but in no case was the effect nearly as 



marked as in Faber's. Even in the case of Falters pencils 



us were made before satisfactory ivsuhs were obtained. 



Equations representing the thermal conditio!, of a bar when 

 acting as a conductor of heat and electricity may be deduced 

 as follows: One end of the bar is supposed to be maintained at 

 a constant temperature, the other at that of the air, and the 

 electric current is supposed to be constant. For simplicity we 

 will assume that the specific electrical resistance of the bar is 

 constant throughout, i. e. is independent of slight differences of 

 temperature. 



The quantity of heat, H, evolved by the current in time 8t, 

 111 the section of the bar S&c— S being the area of a section— is 

 represented by 



H = PRS<fce.<f* I 



' = distance of the section from heated end. If we assume 

 that the thermal conductivity is unaltered by the slight rise in 

 temperature due to the current, it can easily be seen that the 

 flow of heat due to conduction is unaltered by the current. 

 Hence we can consider that the heat evolved by the current is 

 partly used in raising the temperature of the section Sdx, and 

 that all the rest escapes from the surface by radiation. 



The Thomson Effect is, at present, purposely neglected. 



The bar is supposed to have reached a permanent condition 

 ;i ; regarda conduction before the current was passed. Let d be 

 of the section of the bar we are considering 



iityofc 

 L&\p = the rise of temperature above V when the current 

 passes. 

 Assuming Newton's law of cooling, the heat radiated on 

 of the rise of temperature p is proportional to ph, and 

 the quantity radiated from the section in time dt, from the 

 same cause, is 



Hj =phldx.6t II 



' = the periphery of the bar. 



